Silent chains are widely used as timing chains in automobile engines to transmit power from a crankshaft to one or more camshafts, and are also used in many other power transmission applications. A conventional silent chain comprises a large number of interleaved rows of link plates, each plate being bifurcated to form a pair of teeth for engagement with sprocket teeth. The inner engagement surfaces, or “inner flanks,” of the teeth are formed so that they bulge out slightly beyond the outer engagement surfaces, or “outer flanks,” of adjacent, overlapping, teeth in a preceding row of link plates when the chain is straight and under tension. When the chain is used to transmit power, the inner flank starts to engage with a tooth of a sprocket, but the link plate is seated by contact between its outer flanks and a pair of sprocket teeth.
The travel line of a free span of a conventional silent chain approaching a sprocket moves inward and outward relative to the loop formed by the chain. This inward and outward movement is a result of polygonal movement of the chain, also known as “chordal action,” as the chain comes into engagement with the sprocket. As a result of outward movement of the chain, link plates can become inclined slightly forward as they start to engage the sprocket. The sprocket tooth-contacting surfaces of the inner flanks are shaped as circular arcs so that the link teeth slidably contact the sprocket tooth surfaces without excessive friction when starting to engage with, and when separating from, the sprocket. An example of such a chain is shown and described in Japanese Patent Publication No, Hei. 2-19339 for example. Other silent chains have straight inner flanks so that the engagement starting region is straight. However, in the case of chains having straight inner flanks, there is a danger that chordal action and the resulting forward inclination of link plates approaching a sprocket will cause direct clashing of the tips of the link plate teeth with tips of the sprocket teeth.
With the conventional silent chain, even though it is possible to prevent the excessive frictional engagement in starting to engage with the sprocket, contact pressure is high because the contact area immediately after initial engagement of a link plate tooth with a sprocket tooth is small. Consequently, it is difficult to reduce engagement noise.